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Optimisation of propionibacterial ECP production and the influence of propionibacteria on the UASB granulation process

Joubert, Hannarine (2000-12)

Thesis (MSc)--Stellenbosch University, 2000.

Thesis

ENGLISH ABSTRACT: The "classical" propionibacteria are used in a variety of natural dairy fermentations
where they produce natural preservatives (propionic and acetic acids and
bacteriocins) and large amounts of vitamin B12. The extracellular polysaccharide
(ECP) producing ability of these bacteria also make them of special interest to the
food and waste water management industries as the ECP has been illustrated to
playa role in the initial granule formation in upflow anaerobic bioreactor systems.
There is little known on the ECP production by propionibacteria and in this
study different environmental conditions that influence ECP production were studied.
Nineteen different Propionibacterium strains were examined in terms of ECP
production and Propionibacterium strain 278 was identified as the best ECP
producer. Further studies were only done on this strain because of its high ECP
production and because it was originally isolated from an anaerobic digester. The
influence of temperature, pH and sucrose concentration was determined through the
measurement of ECP production and medium viscosity. It was found that more ECP
was produced at temperatures lower than the optimum for growth with the optimum
being between 22° and 25°C. Lower initial pH conditions of the growth medium
(below pH 7.0) were found to inhibit ECP production and the influence when the
initial pH values were between 7.0 and 8.5, was not significant. A higher carbon:
nitrogen ratio, when 8% sucrose was added, was also found to enhance the ECP
production.
The upflow anaerobic sludge bed (UASB) bioreactor process depends on the
upward movement of soluble matter through a blanket of active methanogenic
granular sludge. The long start-up times as a result of the slow granulation process,
as well as the need for a speedy replacement of granules once they have been
washed out of the system, are limitations that restrict the general application of this
excellent waste water treatment technology. Full exploitation of this biomass
immobilisation technique can thus not be realised until the granule formation
conditions are defined and optimised. The precise nature of the mechanisms
involved in the formation of granules and the reason for their stability, is still not fully
understood. It was hypothised by Britz et al. in 1999 that, through the
implementation of environmental 'stress' conditions, a shift in the population dynamics of the anaerobic community can be obtained. This results in a concurrent
increase in ECP formation that appears to enhance aggregate formation.
In the second study it was found that, when 'stress' conditions were applied to
already formed granules, the Gram-positive lactate-utilising acidogenic population
gained an advantage and more propionic acid producing bacteria were present. The
propionic and acetic acid concentrations were also found to increase, and
concurrently, a decrease in the growth medium pH occurred. This confirms part of
the granulation hypothesis that, when granules are 'stressed', the acidogenic
population dynamics change and the lactate-utilising population responds to the
gradual decrease in pH and the more acid-tolerant propionic acid producing bacteria
gain a competitive advantage resulting in the increase in the propionic acid
concentration.
When propionibacteria were added to raw sludge during the granule
production process, the granules were found to be more active than when nopropionibacteria
had been added. This was probably due to the ECP formation by
the propionibacteria that enhances the aggregation of the granules. Enhanced
granulation was thus found in the batch systems with the fatty acids formed in
correlation with the model for granulation. A good correlation was evident between
the hypothesis and the experimental data and the hypothesis was partially verified in
this study.